Why? Because there are two conflicting evolutionary forces at work. In the short-term, mixing mitochondria can be beneficial to individuals because the father’s mitochondria, say, can compensate for a harmful mutation in the mother’s mitochondria. But in the long-term, this can impair evolution’s ability to eliminate bad mutations as they are hidden from view.

Lane thinks this is why organisms have an astonishingly wide variety of mechanisms for ensuring mitochondria are only inherited from the mother. During the course of evolution, species have repeatedly evolved such mechanisms, lost them and then evolved similar mechanisms again, his team has proposed.Michael Le Page, “Some rare fathers pass on an extra kind of DNA to their children” at New Scientist